P89LPC952_954_4 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 04 — 24 July 2008 31 of 69
NXP Semiconductors
P89LPC952/954
8-bit microcontroller with 10-bit ADC
Brownout detection may be enabled or disabled in software.
If brownout detection is enabled the brownout condition occurs when V
DD
falls below the
brownout trip voltage, V
bo
(see Table 11 “Static characteristics”), and is negated when V
DD
rises above V
bo
. If the P89LPC952/954 device is to operate with a power supply that can
be below 2.7 V, BOE should be left in the unprogrammed state so that the device can
operate at 2.4 V, otherwise continuous brownout reset may prevent the device from
operating.
For correct activation of brownout detect, the V
DD
rise and fall times must be observed.
Please see Table 11 “Static characteristics” for specifications.
7.14.2 Power-on detection
The Power-on detect has a function similar to the brownout detect, but is designed to work
as power comes up initially, before the power supply voltage reaches a level where
brownout detect can work. The POF flag in the RSTSRC register is set to indicate an
initial power-up condition. The POF flag will remain set until cleared by software.
7.15 Power reduction modes
The P89LPC952/954 supports three different power reduction modes. These modes are
Idle mode, Power-down mode, and total Power-down mode.
7.15.1 Idle mode
Idle mode leaves peripherals running in order to allow them to activate the processor
when an interrupt is generated. Any enabled interrupt source or reset may terminate Idle
mode.
7.15.2 Power-down mode
The Power-down mode stops the oscillator in order to minimize power consumption. The
P89LPC952/954 exits Power-down mode via any reset, or certain interrupts. In
Power-down mode, the power supply voltage may be reduced to the data retention supply
voltage V
DDR
. This retains the RAM contents at the point where Power-down mode was
entered. SFR contents are not guaranteed after V
DD
has been lowered to V
DDR
, therefore
it is highly recommended to wake-up the processor via reset in this case. V
DD
must be
raised to within the operating range before the Power-down mode is exited.
Some chip functions continue to operate and draw power during Power-down mode,
increasing the total power used during power-down. These include: Brownout detect,
watchdog timer, comparators (note that comparators can be powered down separately),
and RTC/system timer. The internal RC oscillator is disabled unless both the RC oscillator
has been selected as the system clock and the RTC is enabled.
7.15.3 Total Power-down mode
This is the same as Power-down mode except that the brownout detection circuitry and
the voltage comparators are also disabled to conserve additional power. The internal RC
oscillator is disabled unless both the RC oscillator has been selected as the system clock
and the RTC is enabled. If the internal RC oscillator is used to clock the RTC during
power-down, there will be high power consumption. Please use an external low frequency
clock to achieve low power with the RTC running during power-down.
